This invention relates to the treatment of living tissue or cells with electromagnetic fields, and particularly to such treatment employing pulsed electromagnetic fields (PEMFs).
It is well known that time-varying low-energy electric or magnetic fields produce a range of responses in biological systems. In particular, low-energy fields which are essentially athermal are increasingly used for therapeutic purposes as in the healing of fractures and ulcers, assisting bone graft incorporation, etc. The precise mechanisms of action of these low-energy fields are not well understood, but there is evidence that the responses relate to the direct effects of both magnetic and electric fields. Magnetic fields may also act through the induction of electric fields in tissue or cells, and time-varying electric fields may be produced by induction, capacitative coupling or by use of implanted electrodes.
A variety of time-varying electromagnetic waveforms have been studied, patented and used in practice. The widest present application is of electric field inputs to tissue induced by an asymmetrically rising and falling magnetic field as described, e.g., in the following patents:
U.S. Pat. No.InventorIssue Date3,893,462ManningJul. 8, 19754,105,017Ryaby et al.Aug. 8, 19784,266,533Ryaby et al.May 12, 19814,315,503Ryaby et al.Feb. 16, 19824,683,873Cadossi et alAug. 4, 1987
The waveforms of these induced electric inputs are generally described by a quasi-rectangular or trapezoidal-shaped excursion of the electric (E) field followed either by a narrower excursion of higher amplitude in the reverse direction in the case of pulse trains or “bursts,” or by a broad low-amplitude excursion in the case of some repetitive single pulses. It is also known from, e.g., U.S. Pat. No. 4,998,532 to Griffith and U.S. Pat. No. 5,014,699 to Pollack, that more symmetrical pulsed fields produced, for example, by switching abruptly between positive and negative electric fields (achieved, for example, by switching the polarity of the current input to a low inductance coil to give a series of rising and falling magnetic fields in the target tissue or cell system) are also bioactive.
A characteristic of the waveforms in these cases is that abrupt changes occur in the magnitudes or directions of the electric or magnetic fields. For example, in commercial application of the waveforms described by Ryaby et al., the induced electric field rises from zero to its quasi-rectangular “positive” ongoing form or reverses sign to a higher-amplitude narrow “negative” excursion in a time interval of about one microsecond or less, as compared to pulse widths typically on the order of tens to thousands of microseconds. Correspondingly, the time derivative of the magnetic field changes sign over an interval of about one microsecond, whereas the intervals in which the sign is unchanged are typically ten or more times as long. In the work of Griffith and Pollack, the preferred pulse widths are on the order of ten microseconds, these pulses being repeated in bursts on the order of milliseconds. The reversal time for these pulses is typically one microsecond or less. Trains of pulses as in the above-referenced Ryaby et al. patents, all of which are hereby incorporated by reference, typically last for milliseconds and the pulses or pulse trains are repeated at intervals on the order of tens to thousands of milliseconds. Still, in commercially available equipment generating such pulse trains, the rise and fall times for individual pulses are abrupt, i.e., about one microsecond or less.
In some cases, the abrupt changes in the time derivative of the electric field may be associated with overshooting, which may include a damped oscillation or ringing on the order of 1 MHz, i.e., within the range of about 0.5 to 5 MHz. The frequency content of asymmetric pulse waveforms of the Ryaby et al. type, as derived by discrete Fourier Transforms, can range to 10 MHz, as described by Goodman et al. in a paper entitled “Exposure of Human Cells to Low-Frequency Electromagnetic Fields Results in Quantitative Changes in Transcripts,” in Biochimica et Biophysica Acta, 1009 (1989) 216–220.
The bioresponse of PEMF signals with rapidly changing pulses can be improved by signal shaping to reduce the high-frequency components of the signals, as described in U.S. Pat. No. 5,338,286 to Abbott et al., which patent is hereby incorporated by reference. In terms of the time domain, significant improvement in the bioefficacy of low-energy pulsed electromagnetic signals configured with waveforms ordinarily characterized by abrupt changes in magnitude or sign of the electric or magnetic field is achievable by reducing or eliminating one or more of the previously characteristic abrupt changes and providing smoother transitions to relatively flat segments or portions of the signal or waveform. “Abrupt” in this context is intended to connote time intervals of approximately 1 microsecond or less.